The histocompatibility-2 (H-2) complex of the mouse consists of a series of closely linked loci on chromosome 17 which control protein and cell membrane antigens involved in a number of critical functions in cellular immunity. This proposal suggests a program for expanding our knowledge of the genetic organization of the H-2 complex and for dissecting and defining the genetic control of the many important immunological phenomena associated with this gene complex. The primary approach involves the development of a large number of intra-H-2 recombinants from a single heterozygote combination (H-2b/H-2tl) to provide two series of recombinant strains which are genetically identical except for a portion of chromosome within the H-2 complex.